MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AGE HARDENABLE Al-ALLOY PROCESSED BY CRYOROLLING AND CRYOFORGING

Abstract

Now a days. a top—down technique like severe plastic deformation has become a new way of transforming conventional grained materials to ultrafine grained materials in bulk. Although most of the processes improve strength at the cost of' ductility or vice versa, a few most recent approaches have succeeded in obtaining simultaneously high strength and reasonable high ductility. 'Iberefore, airn of the present study is to give basic insight into the field of ultrafinc-grained 2024 Al-alloy obtained by multiaxial forging (MAF) and cryorolling at cryogenic temperature. In the present work, commercially available Al 2024 alloy is subjected to multiaxial forging at cryogenic temperature and cryorolling. The evolution of ultrafiuie grain structure during MAF at various strain levels (AcI.2 and ZAc =2.4) is investigated using optical microscopy. Formation of ultrafme grained structure with high angle grain boundaries through continuous dynamic recrystallization is observed with increasing strain up to (Ac=2.4). Both multiaxially lbrged (MAFed) and ctyoro lied samples show an improvement in YS and UTS but ductility decreases. Aging treatment of' the MAFed samples at optimum temperature and time increases not only the yield strength, but also its ductility. Fractography analysis revealed a quasi-cleavage fracture of the MAFed±aged sample, whereas, brittle failure could be observed for the MAFed specimen. Corrosion behaviour of' the alloy produced by MAF+ageing is investigated with respect to the coarse grained counterpart. Corrosion resistance of the MAl•'ed±aged sample found to be higher compared to MAFed and coarse grained specimens. The depletion of Cu and Mg from the matrix due to the formation of strengthening precipitates on aging increases the resistance to corrosion. Also more amount of' grain boundary area of the fine grained structure reduces the chloride concentration per grain boundaiy resulting in lesser current density.